Known boiling water nuclear reactors are provided with a large, central core. In the typical construction, liquid water coolant/moderator flow enters the core from the bottom and exits the core as a water steam mixture from the top. The core includes many side-by-side fuel bundles. Water is introduced into each fuel bundle through a fuel bundle support casting from a high pressure plenum, which is situated below the core. Water passes in a distributed flow through the individual fuel bundles, is heated to generate steam, and exits the upper portion of the core as a two phase water steam mixture from which the steam is extracted for the generation of energy.
Typically, each fuel bundle includes a matrix of upstanding fuel rods. The fuel rods are sealed tubes, each containing fissionable material, which when undergoing a nuclear reaction, produce power generating steam. At the upper end of the matrix of upstanding fuel rods is located a so-called upper tie plate. This upper tie plate holds at least some of the fuel rods in vertical side-by-side alignment. Some of the fuel rods may be attached to both the upper tie plate and corresponding lower tie plates. Between the upper and lower tie plates, there are generally included water rods (or equivalent devices) for improvement of the water moderator to fuel ratio, particularly in the upper, highest void fraction region of the fuel bundle.
In addition, fuel bundles also include about seven or eight fuel rod spacers at varying elevations along the length of the fuel bundle. These spacers are required because the fuel rods are long (about 160 inches) and slender (about 0.4 to 0.5 inches in diameter), and would come into contact under the dynamics of fluid flow and nuclear power generation within the fuel bundles. The spacers are normally in the form of grid and include a plurality of individual spacer cells that provide appropriate restraints for each fuel rod at their respective elevations and thus prevent contact between the fuel rods and maintain the fuel rods at uniform spacing relative to one another along the length of the fuel bundle for optimum performance. Further, adjacent spacer cells are commonly joined by a common spring that biases adjacent fuel rods in opposite directions toward stops on the spacer cell.
It has been found that it is generally desirable to form the cladding tube from a zirconium-based alloy while in newer spacer designs, the spacer and stop materials are preferably formed from a nickel-based alloy, e.g. Inconel or X-750, or a Fe-based alloy, e.g. 304 SS. It has been occasionally observed that that a nickel-based alloy spacer exhibits radiation-enhanced corrosion, e.g. shadow corrosion, when a zirconium-based material is positioned adjacent thereto. This enhanced corrosion is manifested by a so-called “shadow effect” that takes the form of a region of enhanced corrosion on the zirconium-based component that occurs immediately adjacent to the shadowing component.